R Coronae Borealis (RCB) and dustless Hydrogen-deficient Carbon (dLHdC) Historically, dLHdC stars were referred to as HdC stars. Here, we follow the updated nomenclature of Tisserand et al. 2021. stars are supergiants characterized by a peculiar chemical composition – an acute deficiency of hydrogen and an overabundance of carbon [1]}, [2]}. Together, they form the class of Hydrogen-deficient Carbon (HdC) stars. Their chemical compositions suggest that they are remnants of a He-core and a CO-core white dwarf (WD) merger [3]}, [4]}, making them low mass counterparts of type Ia supernovae in the double-degenerate (DD) scenario [5]}. In addition to peculiar chemical compositions, RCB stars experience rapid photometric declines (up to 8 mag in V band) that are attributed to dust formation episodes [4]}. dLHdC stars do not show any such declines or any significant infrared (IR) excess, suggesting that they do not undergo any dust formation [4]}. Why RCB stars produce dust while dLHdC stars do not, despite having no other known chemical differences is still a mystery.
